בראשית, פרק א׳, פסוק י״ג

The concept of time and the definition of a day take shape at the very dawn of existence. The natural cycle moving from darkness to light establishes the foundation for the calendar and the basic rhythm of life. A full day is defined as a twenty-four-hour period that begins at nightfall and lasts until the following night. This fundamental structure is the reason why the Sabbath and all holidays always begin on the preceding evening [חומש קה״ת].

This early establishment of time presents a logical puzzle regarding the first three days of creation. Naturally, evening occurs when light sets, and morning happens when light rises. Yet, the sun and moon were not created until the fourth day. During the initial three days, the cycle of evening and morning did not depend on sunlight. Instead, it was based on the rotation of the celestial sphere itself. As the firmament revolved, every part of it experienced a morning as it rotated upward and an evening as it rotated downward. It is only from the fourth day onward that these times of day became linked to the actual light of the heavenly bodies [רבנו בחיי].